Is Australopithecus afarensis Too Derived to be a Human Ancestor

My reaction on first hearing about this paper.
Having gotten that out of the way, I sat down and read the paper – which was published in PNAS. The paper was written by Yoel Rak, Avishag Ginzburg, and Eli Geffen. The short version is that Australopithecus afarensis displays mandibular traits that indicate it is too derived to be a human ancestor. The longer version is a little bit more interesting, although I am inclined to be somewhat skeptical.

Let’s start at the beginning. Below is a picture of a human mandible:
The anatomy we are interested in is in the upper right hand corner of the bottom picture. Specifically, the coronoid process, the mandibular notch and the condyle. In humans, the condyle is taller than the coronoid process and the deepest part of the mandibular notch is situated anteriorly (towards the front of the jaw). According to Aiello and Dean’s An Introduction to Human Evolutionary Anatomy the reverse is true in apes. Rak et al take a slightly different view. The analyzed the shape of the mandible in 146 extant primates, including gorillas, chimps (common and pygmy), orangutans and humans. According to their analysis, the above species fell into two groups based on their mandibular anatomy. The first group includes humans, chimps and orangs. The second consisted solely of the gorilla. Then they threw some fossil specimens into the mix. The fossil specimens include the recently (2002) discovered A. L. 822-1, MAK-VP 1/83 (both Au. afarensis), SK 23, Sk 34 (both Au. robustus) and GWM5sw/P56 (Ardipithecus ramidus). Some of which are pictured below (along with a key):
Afarensis%201.JPG (Ramal morphology in Au. afarensis and extant primates. (Top) Left mandibular ramus and right mandibular ramus (horizontally flipped) of Au. afarensis specimen A. L. 822-1 and left mandibular ramus of a gorilla. (Middle) Left mandibular ramus of Au. afarensis MAK-VP 1/83 specimen; fragment of left mandibular ramus of Au. afarensis specimen A. L. 333-100; and mandibular ramus of Au. robustus specimen SK 23. (Bottom) Mandibular ramus of a chimpanzee, an orangutan, and H. sapiens. (Scale bar: 5 cm.) Note that the upper end of the ramus in all of the specimens above the white line resembles that of a gorilla (particularly in the shape of the coronoid, the great percentage that the coronoid base constitutes of the ramal width, the confined appearance of the mandibular notch, and the small percentage that the notch area constitutes of the ramal area). The limited reconstruction of the coronoid process on the left ramus of A. L. 822-1 is based on the corresponding preserved area on the right ramus and vice versa.)
Rak et al argue that the condition seen in chimps, humans and orangs represents the primitive condition and that seen in gorillas and Au. afarensis are therefore derived. Furthermore, they argue that the condition seen in Au. afarensis is too similar to that seen in Au. robustus for Au. afarensis to be a direct human ancestor. To support that contention they also point out that the condition in Ardipithecus ramidus is similar to chimps, humans and orangs. Interestingly enough, the Dikika find displayed some gorilla-like traits.
Having said all that, I am somewhat skeptical. One reason is that Au. afarensis has always been considered ancestral to both humans and robust australopithecines, so the fact that Rak et al found another trait that connects them doesn’t surprise me. Nor does it, in my opinion, rule them out as human ancestors. What it does do is reinforce the highly transitional nature of Au. afarensis. Another reason is that starting with the announcement of the discovery of Au. afarensis there have been a number of suggestions that the morphology is so varied that two species must be represented. This paper even argues that the assemblage represents two species – a hominid and a pongid (similar to Dryopithecus). The fact that Au. afarensis has some traits similar to the gorilla doesn’t really surprise me (I would have been surprised if they had orang or gibbon traits). I’ll repeat what I said in a previous post:

This issue of taxonomic blurring is only going to loom larger in paleoanthropology as more fossils are discovered. Think of the reptile/mammal transition where deciding whether a given specimen is more properly considered a reptile or a mammal can be difficult (technically this is a somewhat misleading way of saying it. Mammals and mammal like reptiles are members of the Synapsida and technically mammal like reptiles are called nonmammalian synapsids and the nonmamalian synapsids share some primitive features with the common ancestors of reptiles and synapsids. Essentially, the reptiles and synapsids form a sister group relationship rather than an ancestor/descendant relationship.) The point here is that as the hominid family tree proliferates we are going to see a wide variety of fossils with a wide variety of traits that are going to blur the boundaries not only between Australopithecus and Homo but within those taxonomic categories as well.

Or, in this case, between members of the Hominidae, but then I could be in denial…

6 Responses

  1. Doesn’t that thicker coronoid process relate directly to the chewing behavior of the animal?
    I think there’s a fairly good chance that this finding tells us more about the diet of afarensis rather than its evolutionary relationships.

  2. Rak et al admit this could be a homoplasy between gorillas, afarensis, and the robusts. They argue that:

    …even if the presence of similar ramal
    morphology in Au. afarensis and Au. robustus did, indeed,
    represent homoplasy, the Au. afarensis ramal anatomy would still
    exclude this taxon from our ancestry.

    I, personally, think there is some hand waving going on…Of course they also don’t have an explanation for why the orangs group with ardipithecus, chimps and humans. (incidentally, africanus groups with afarensis, the robusts and gorillas).

  3. I was looking, but I didn’t see that you commented on this paper (described in the April 7 issue of Science):
    Carol Ward of Univ. Missouri, Columbia seems convinced from wrist bones that the “Hobbit” is not a homo sapien, but a new species. What is your take on this?
    Hobbit’s Status as a New Species Gets a Hand Up
    Ann Gibbons
    PHILADELPHIA, PENNSYLVANIA–The diminutive human who lived on the Indonesian island of Flores 18,000 years ago has been called many things: a pygmy, a diseased Homo sapiens, a hobbit. Now, in a report that was the talk of the Paleoanthropology Society’s annual meeting here last week, a postdoctoral researcher claimed that the shapes of the fossil’s wrist bones are so primitive that it cannot be H. sapiens. “It is definitely not a modern human. It’s not even close,” paleoanthropologist Matthew Tocheri of the Smithsonian Institution in Washington, D.C., said in his talk. Although some critics still think the bones could be those of a diseased H. sapiens, others who heard Tocheri’s report were persuaded. “It’s the most convincing evidence so far that it really is something different,” says paleoanthropologist Carol Ward of the University of Missouri, Columbia.

  4. I have not read that paper (it’s not open access). Hawks mentions that presentation.

  5. I read the Rak et al. paper and it’s an interesting analysis of one character. But one character – unless it’s the only one available for one or more of the taxa concerned – is NEVER enough to make phylogenetic conclusions. Homoplasy is too common. A few years ago I thought that palaeoanthropology was finally coming inside the tent with (most of) the rest of the vertebrate palaeontology and systematics crowd, and starting to analyse actual data matrices. I saw a few papers like that (real phylogenetic analyses, from my point of view), but can’t recall any recently. Didn’t it work out, or have I been looking at the wrong journals?

  6. John,
    That is actually a very good point. I remember seeing multi-character analyses of large data sets awhile back (say late 90’s early 2000’s), but nothing recently (that has come to my attention anyway – I’m going to have to do some checking on that). It’s a pity, because some of the papers I read looked promising. Certainly there are enough anthropologists out there with the requisite skills to do that kind of phylogenetic analysis. as far as homoplasy goes, there is tons of it in primate evolution – which I think may actually be a stumbling block in anthropology.

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